Atherothrombosis, resulting from rupture or erosion of unstable atherosclerotic plaques, is the leading cause of death worldwide. However, the mechanisms that regulate the stability of late stage atherosclerotic lesions remain poorly understood. The dogma is that: 1) plaques containing a large necrotic core, a thin fibrous cap, and large numbers of CD68+ cells relative to Acta2+ cells [presumed to be macrophages (M?) and smooth muscle cells (SMC) respectively] are more prone to rupture; and 2) SMC play a beneficial role because they are the primary cell type responsible for formation of a matrix-rich protective fibrous cap. However, recent studies by our lab involving simultaneous SMC lineage tracing and SMC-specific knockout (KO) of the stem cell pluripotency genes Oct4 or Klf4, provide compelling evidence challenging this dogma including showing that SMC can have major beneficial or detrimental effects on late stage lesion pathogenesis depending on the nature of their phenotypic transitions. As such, we sought to identify mechanisms to promote beneficial (atheroprotective) SMC phenotypic transitions, and hypothesized that treatment with an anti-IL1? antibody (Ab) to globally suppress inflammation would induce such changes. However, completely contrary to expectations, treatment of our SMC lineage-tracing ApoE-/- mice with the anti-IL1? Ab between 18-26 weeks of Western diet resulted in multiple detrimental changes including a marked reduction in fibrous cap thickness and collagen content, rapid loss of fibrous cap SMC and replacement with M?s, and impaired outward remodeling. Studies in this proposal will test the hypothesis that that IL1? signaling in SMC plays a critical beneficial role in late stage lesion pathogenesis including being required for formation and maintenance of a protective fibrous cap. Aim 1 will determine if persistent IL1R1 signaling within SMC is required for maintenance of a SMC rich fibrous cap. Studies will include determining the contributions of each of the IL1R1 ligands IL1?, and IL1?, as well as IL1Ra. We will also determine if SMC- specific tamoxifen conditional KO of the IL1R1 receptor, after establishment of advanced atherosclerosis by 16- 18 weeks of WD feeding, results in loss of SMC fibrous cap coverage and other detrimental changes consistent with plaque destabilization. Finally, we will determine if IL1? dependent phenotypic changes of SMC observed in our mouse studies occur in human lesions and if these changes are predictive of plaque rupture or erosion. Aim 2 will define critical variables and mechanisms that influence the effects of IL1? neutralization on late stage lesion pathogenesis including those that may help to reconcile our findings with the recent positive outcomes of the CANTOS Clinical Trial. This will include testing the hypotheses that the response to anti-IL1? therapy is highly dependent on whether there is persistent hyperlipidemia/unresolved inflammation, and/or concurrent myocardial infarction/heart failure. Taken together, the proposed studies will provide key insights regarding fundamental mechanisms that regulate late stage lesion pathogenesis, and may contribute to development of improved therapies for treating patients with advanced atherosclerosis.